Traditional semiconductor memory devices store a memory state by storing an electronic charge. However, magnetoresistive random access memory (hereinafter referred to as “MRAM”) devices store a memory state by utilizing the direction of the magnetic moment vector created in a magnetic material or structure. Thus, a memory state in a MRAM device is not maintained by power, but rather by the direction of the magnetic moment vector. To be commercially viable, however, MRAM must have comparable memory density to current memory technologies, be scalable for future generations, operate at low voltages, have low power consumption, and have competitive read/write speeds.
In previous MRAM technology, storing data is accomplished by applying magnetic fields and causing a magnetic material in a MRAM device to be magnetized into either of two possible memory states. Thus, a single MRAM device typically stores one bit of information and to increase the memory density, the MRAM device must be scaled laterally to smaller dimensions.
As the bit dimension shrinks, however, three problems occur. First, the switching field increases for a given shape and film thickness, requiring more current to switch. Second, the total switching volume is reduced so that the energy barrier for reversal, which is proportional to volume and switching field, drops. The energy barrier refers to the amount of energy needed to switch the magnetic moment vector from one state to the other. The energy barrier determines the data retention and error rate of the MRAM device and unintended reversals can occur due to thermal fluctuations if the barrier is too small. Finally, because the switching field is produced by shape, the switching field becomes more sensitive to shape variations as the bit shrinks in size. With photolithography scaling becoming more difficult at smaller dimensions, MRAM devices will have difficulty maintaining tight switching distributions.
Accordingly, it is an object of the present invention to provide a new and improved magnetoresistive random access memory device which can store multiple states.